This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from my application OPTICAL WAVEGUIDE AND METHOD FOR FABRICATING THE SAME filed with the Korean Industrial Property Office on Jan. 18, 1999 and there duly assigned Ser. No. 1261/1999.
The present invention relates to an optical waveguide, and more particularly to an optical waveguide having a specific dopant distribution capable of reducing a deformation thereof and existence of residual stresses therein, thereby involving no birefrigence. The present invention also relates to a method for fabricating such an optical waveguide.
Optical waveguides are transmission paths adapted to propagate optical waves in a longitudinal direction while confining those optical waves within a certain cross section, thereby reducing propagation losses of the optical waves. Typically, such an optical waveguide includes a core, and a clad exhibiting a refractive index lower than that of the core.
In order to fabricate such an optical waveguide, a flame hydrolysis deposition method is most commonly used in which a lower clad layer, a core layer, and an upper clad layer are sequentially formed over a substrate.
In accordance with the flame hydrolysis deposition method, a soot is deposited over a substrate by oxidizing and hydrolyzing a chemical substance such as SiCl4, GeCl4, POCl3, BCl3 or the like using oxygen and hydrogen flames. The deposited soot is then sintered at a high temperature in a sintering furnace to obtain a transparent silica film.
In order to reduce such an induced birefringence, a variety of methods have been proposed. One method is to form an asymmetric stress-releasing groove. Another method is to deposit a stress applying member on a clad thin film. Another method has been proposed in which a buffer layer is interposed between a substrate and a lower clad layer.
The first method should use an etching process whereas the second method requires formation of an additional stress applying layer made of silicon or Si4N4 over the entire portion of the clad thin film or on an intermediate portion of the clad thin film. In the case using a buffer layer, processes for depositing and sintering the buffer layer should be used. Due to such processes, the processing time is lengthened, thereby resulting in an increase in the production costs.
U.S. Pat. No. 4,339,174 for a High Bandwidth Optical Waveguide to Levin discloses an optical waveguide having a cladding layer, a core layer, and a barrier layer disposed between the core and the cladding. Dashed line 16 in FIG. 1 shows B2O3 concentration being graded from the barrier layer level to zero at a radius rd. U.S. Pat. No. 4,248,614 for a Method For Drawing High-Bandwidth Optical Waveguides to Scherer and U.S. Pat. No. 4,181,403 for an Optical Fiber With Compression Surface Layer to Macedo et al and U.S. Pat. No. 4,230,396 for a High Bandwidth Optical Waveguide And Method of Fabrication to Olshansky et al disclose waveguides where the refractive index and dopant concentration vary within the waveguide. However, I have noticed that these references do not disclose buffer layers within the waveguide. In addition, these references pertain only to optical waveguides of a circular cross section. What is needed is an optical waveguide with a rectangular cross section that contains buffer layers that varies the dopant concentration in a stepwise and a linear fashion.
Therefore, an object of the invention is to provide an optical waveguide including a lower clad layer formed over a silicon substrate while being doped with a dopant to reduce a thermal expansion coefficient difference between the silicon substrate and the lower clad layer and to reduce the glassification temperature of the lower clad layer, thereby preventing the silicon substrate from being deformed and reducing existence of residual stresses.
Another object of the invention is to provide a method for fabricating the optical waveguide.
It is yet another object to provide a waveguide having buffer layers.
It is still another object to provide a waveguide having a rectangular cross section.
It is still yet another object to provide an optical waveguide with a rectangular cross section and to decrease polarization dependent loss (PDL) owing to the rectangular shape of the waveguide.
In accordance with one aspect, the present invention provides an optical waveguide comprising: a substrate; a lower clad layer doped with a dopant in a content varying continuously in a thickness direction of the lower clad layer, so that the lower clad layer exhibits a refractive index distribution varying in the thickness direction thereof; and a core layer formed over the lower clad layer.
In accordance with another aspect, the present invention provides a method for fabricating an optical waveguide comprising the steps of: (a) depositing a lower clad layer over a substrate while continuously varying a dosage of a dopant to be doped in the lower clad layer in such a fashion that the lower clad layer exhibits a refractive index distribution varying in the thickness direction thereof; and (b) forming a core layer over the lower clad layer.